Conventionally, there has been disclosed a method of manufacturing an electrode of a solar cell, comprising: forming a lower electrode layer by coating and baking a first solution coated on a photoelectric conversion semiconductor layer; and forming an upper electrode layer by coating and baking a second solution on a photoelectric conversion semiconductor layer. In this method, the first solution includes ultrafine metal particles having a grain size of 0.03 μm or less which are dispersed in an organic solvent having a low molecular number of about 100 to 200, and the second solution includes ultrafine metal particles in the same or higher weight concentration than in the first solution used in the above-described formation of the lower electrode layer (For example, see Claim 1, paragraph [0024], and paragraph [0035] of Patent Reference 1: Japanese Patent No. 3287754 which is hereby incorporated by reference herein in its entirety). In this method of manufacturing metal electrodes, ultrafine metal particles are dispersed in the solution, and the viscosity of the solution is controlled to be about 10000 cps. The solution is coated on the photoelectric conversion semiconductor layer by a screen printing method or the like. After that, the metal electrode (the upper electrode layer or the lower electrode layer) is formed by baking the solution for 30 minutes or more at 100 to 250° C., and preferably at 250° C.
In the thus constituted method of manufacturing metal electrodes of a solar cell, the solution dispersing ultrafine metal particles in the organic solvent is coated on the photoelectric conversion semiconductor layer, and subsequently the solution is baked at a low temperature of 100 to 250° C. By this method, it is possible to obtain metal electrodes having high reflectance, high conductivity, and large area without using high-vacuum process.
An electric-conductive composition consisting of Ag powder; at least one metal selected from V, Mo, and W; glass frit; and an organic vehicle is disclosed as a composition used in a method of manufacturing electrodes of a solar cell (for example, see claim 2, paragraph [0022] and paragraph [0031] of Patent Reference 2: Japanese Unexamined Patent Application, First Publication, No. Hei 10-326522 which is hereby incorporated by reference herein in its entirety). In the above-described Patent Reference 2, a base material printed with the electric-conductive composition for a solar cell is subjected to baking for five minutes at 550° C., thereby forming a Ag electrode. By using the electric-conductive composition for a solar cell described in Patent Reference 2, it is possible to remarkably enhance the sinterability of an Ag electrode. Specifically, it is possible to improve the electric conductivity and film-strength of an electrode under a low-temperature baking at 700° C. or less. Therefore, it is possible to contribute to cost-reduction by decreasing the baking temperature, and formation of electrodes in a case where a treatment temperature of a substrate element is restricted by an upper limit.
Patent Reference 3 discloses an electric-conductive paste formed by mixing a metal powder, an oxide powder, and a vehicle. The metal powder is a powder of at least one metal selected from a group consisting of Ag, Cu, and Ni. The oxide powder is a crystalline powder of a complex oxide that includes at least one selected from a group consisting of Bi, Fe, and Ag, and at least one selected from group V elements and group VI elements of a periodic table (for example, see claims 1 to 3, paragraph [0009], [0031] of Patent Reference 3: Japanese Unexamined Patent Application, First Publication, No. H11-329070 which is hereby incorporated by reference herein in its entirety). In the above-described Patent Reference 3, an electrode is formed by baking a wafer printed with the electric-conductive paste, where the maximum baking temperature is 750° C. By using the electric-conductive paste described in the Patent Reference 3, it is possible to form an electrode having low contact resistivity and high bonding-strength certainly.
Further, Patent Reference 4 discloses a method of manufacturing a solar cell element comprising: forming an area of one type electric-conductivity on one main surface of a semiconductor substrate that provides another type conductivity; forming an anti-reflection film on the main surface of the semiconductor substrate; and bake-printing a raw material of an electrode on another main surface of the semiconductor substrate opposite to the anti-reflection film, wherein the raw material comprises silver powder, an organic vehicle and glass frit, and the raw material further comprises one or a plurality of species selected from Ti, Bi, Co, Zn, Zr, Fe, and Cr components (for example, see Claim 1, paragraph [0027], paragraph [0039] of Patent Reference 4: Japanese Unexamined Patent Application, First Publication, No. 2001-313400 which is hereby incorporated by reference herein in its entirety). In the above-described Patent Reference 4, a solar cell element is formed by bake-printing the paste at 700° C. In the method shown in the Patent Reference 4, even when the raw material of an electrode is coated and baked on the anti-reflection film, it is possible to obtain a solar cell element having a satisfactory ohomic contact (fill factor), and a strong tensile-strength.
A method of forming an electric-conductive coating by coating and heating an electric-conductive composition has been conventionally disclosed as a method of forming a metal electrode on a semiconductor substrate using a raw material containing fine metal particles, for example, an electric-conductive paste. In this conventional method, the electric-conductive composition contains particulate silver compounds, reducing agent and a binder, where the silver compounds may be silver oxide, silver carbonate, and silver acetate or the like (for example, see claims 1 to 3, 11, lines 32-33 on page 3 of Patent Reference 5: PCT International Patent Publication No. 2003/085052 which is hereby incorporated by reference herein in its entirety). According to the method of Patent Reference 5, without using a high-temperature film formation conditions, it is possible to obtain an electric-conductive film of low volumetric resistivity and high conductivity similar to those of metallic silver.
There has been disclosed an electric-conductive paste and a method of manufacturing a solar cell using the electric-conductive paste. The electric-conductive paste comprises an organic binder, a solvent, glass frit, and an electric-conductive powder, wherein the paste contains a powder of metal or metal compound including at least one selected from Ti, Bi, Zn, Y, In, and Mo, and having an average grain size of not less than 0.001 μm and less than 0.1 μm. The solar cell is produced by printing or coating the electric-conductive paste on an anti-reflection layer of a silicon semiconductor substrate, and subsequently baking the substrate. (For example, see claims 1, 6, paragraphs [0021] of Patent Reference 6: Japanese Unexamined Patent application, First Publication No. 2005-243500 which is hereby incorporated by reference in its entirety). In the electric-conductive paste shown in the above-described Patent reference 6, it is preferred to bake the substrate printed or coated with the electric-conductive paste at a temperature of 550 to 850° C., thereby forming the electrode. According to the Patent Reference 6, additive of ultra fine particles are uniformly dispersed. By baking the substrate, it is possible to form a surface electrode having high conductivity and superior bonding strength between the semiconductor and the electric-conductive paste interposing the anti-reflection layer.
In the conventional method of forming metal electrodes of a solar cell shown in the above-described patent reference 1, in order to stabilize the ultrafine metal particles in the metal electrode after the baking, it is necessary to protect the ultrafine metal particles by an organic material having a low molecular weight of about 100 to 200 while ensuring a predetermined electric conductivity. On the other hand, when the size of the ultra-fine metal particles is reduced so as to sinter the ultra-fine metal particles dispersed in the organic solvent at a low temperature, the proportion of the organic material is increased because of increasing specific surface area of the ultra-fine metal particles.
Therefore, in the above-described conventional method of forming metal-electrodes of a solar cell described in the above-described patent reference 1, low-temperature sintering of the ultra-fine metal particles dispersed in the organic solvent cannot be realized without desorbing or decomposing (isolating·combusting) the organic material. Where the metal-electrode obtained by baking the ultra-fine metal particles dispersed in the organic solvent at 220° C. or less is subjected to a weathering test by storing the metal electrode for 1000 hours in a thermo-hygrostat while retaining the temperature at 100° C. and the humidity of 50%, there has been a problem of reduction of electric conductivity and reflectance caused by the alteration or deterioration of the organic material.
As shown in the above-described Patent References 2 to 4, where the base material is made of a silicon substrate, ceramic substrate, or a glass substrate, it is possible to form a film with high bonding strength by using a high-temperature baked type thick-film paste that utilizes glass frit or its alternative material so as to enhance the bonding strength to the substrate. However, the compositions forming the electrodes described in the patent references 2 to 4 have to be baked at a temperature of not lower than 500° C. At that temperature, there has been a problem of harming the base material.
Where a base material is made of macromolecule substrate such as an organic polymer substrate, an electric-conductive adhesive containing an organic-based adhesive or a low-temperature polymer type thick film paste containing an organic binder is also used to enhance bonding strength. In this type of paste, electric conductivity is obtained by baking at 200° C. or less. By this baking, thermal shrinkage of the binder occurs making the electric-conductive fine particles contact with each other. However, there have been problems. For example, the presence of an insulating binder in the interstices of the particles results in a large component of contact resistance between the electric-conductive particles, causing high volume resistivity of the formed electrode, and keeping the electric conductivity at low value.
In the method of manufacturing electrodes of a solar cell shown in the above-described Patent Reference 5, it is possible to achieve an electrode made of metallic film having a volume resistivity close to a bulk metal. However, it is difficult to obtain a film having a high adhesion to the base material.
In addition, in the method shown in the above-described Patent Reference 6, it is necessary to melt the glass frit. Therefore, it is necessary to perform the baking at a temperature of not lower than 300° C., that is, a softening temperature of a borosilicate glass applied as a representative glass frit. The preferred baking temperature is also high in the Patent Reference 6. Therefore, for example, in the case of bonding to an amorphous silicon base material of a solar cell, there are problems such as deterioration of conversion efficiency. In addition, since the baking temperature is higher than a heatproof temperature of most of resins, it is difficult to apply the method to base materials basically made of resins.